Merge pull request w3c#386 from chrishtr/fixcompositing

Fix text to specify correct blend-mode on the root element

Author: chrishtr

compositing-1/Overview.bs

@@ -82,6 +82,8 @@ Everything in CSS that creates a <a href="https://www.w3.org/TR/CSS21/zindex.htm
 
 An element that has blending applied, must blend with all the underlying content of the <a spec="css21">stacking context</a> [[!CSS21]] that that element belongs to.
 
+The root element for an HTML document is the <a href="https://dom.spec.whatwg.org/#document-element">document element</a>.
+
 <h3 id="csscompositingrules_SVG">Behavior specific to SVG</h3>
 
 By default, every element must create a <a href="#isolatedgroups">non-isolated group</a>.
@@ -98,6 +100,8 @@ However, certain operations in SVG will create <a href="#isolatedgroups">isolate
 <li>masking</li>
 </ul>
 
+The root element for SVG is the <a href="https://www.w3.org/TR/SVG11/struct.html#NewDocument">SVG element</a>.
+
 <h3 id="csskeywords">CSS Properties</h3>
 
 <h4 id="mix-blend-mode">The 'mix-blend-mode' property</h4>
@@ -700,21 +704,19 @@ On the left, the group has the 'knock-out' property activated. On the right, the
 If the 'knock-out' property is disabled, each element within the group is only composited with the elements underneath the group.
 </p>
 -->
-<h3 id="pagebackdrop">The Page Group</h3>
+<h3 id="pagebackdrop">The Root Element Group</h3>
 
-The <a href="#isolatedgroups">isolated group</a for the root element is the
-page group. All other elements and groups are composited into this group.
-The background of the root element (if specified) is painted into the page
-group, and any filter, clip-path, mask and and opacity is then applied, before
-compositing into the <a href="#rootgroup">root group</a>.
+The <a href="#isolatedgroups">isolated group</a> for the root element is the
+root element group. All other elements and groups are composited into this group. The background of the root element (if specified) is painted into the
+root element group, and any filter, clip-path, mask and and opacity is then
+applied, before compositing into the <a href="#rootgroup">root group</a>,
+if present.
 
 <h3 id="rootgroup">The Root Group</h3>
-The root group is the final compositing output, and is is above the page
-group. Compositing of the page group into the root group is performed with the
-specified blend mode of the root element.
+The root group encompasses the entire canvas and contains (or is below) the root element group of the root element of a web page.
 
-<p class="note">Browsers often use an infinite white, 100% opaque root group,
-for final compositing, but this is not required. One example is an iframe with a transparent background.
+<p class="note">Browsers often use an infinite <a href="https://www.w3.org/TR/css-color-4/#sample">white, 100% opaque</a> root group,
+for final compositing, but this is not required.
 
 <h2 id="advancedcompositing">Advanced compositing features</h2>